Spend some time doing research, not just about the BP, but how to take care of it. Show her that you are really interested and committed to taking care of the snake. Plan out what you would want to do for a cage or tub, including heating, hides, a thermometer and humidity gauge, feeding, etc.... Give her a cost breakdown of everything it will take to get set up and care for your snake properly.
I know if one of my kids took that much time and effort, I would certainly consider and listen to what they had to say.

The other reason I say all this is for YOU to be sure you want to take on the responsibility of owning a BP.

Good Luck

Neil